
No fundamental diagram for simulation purposes. Before being implemented in practice any ramp metering procedure ought to be tested via computers. The similarities between highway traffic and hydrodynamics explain the importance of the Lighthill-Whitham-Richards (LWR) first order partial differential equation [1]}, [2]}. Its numerical integration necessitates to know the relationship between the traffic density and the traffic speed. Several empirical laws, called fundamental diagrams, have been proposed. The calibration of those diagrams is far from being obvious. That is why contrarily to other approaches (see, e.g., [3]}) we ignore them. Fundamental diagrams are replaced by traffic data which are collected during various situations. Note moreover that our viewpoint, contrarily to other ones like METANET [4]}, does not necessitate higher order partial differential equations.

No difficult estimation technique. Implementing any control law requires crucial quantities like the critical density, which are time-varying and therefore almost impossible to estimate in real time. They are replaced by ad hoc quantities which are deduced at once from easily measurable data.

Model-free control. ALINEAIt is the acronym of Asservissement LINéaire d'Entrée Autoroutière. [5]}, [6]}, [7]}, [8]} is perhaps the most popular feedback control algorithm for traffic regulation. After first trials in Paris it has been quite often employed in many different places. Following [9]}, we are using here model-free control in the sense of [10]}, [11]} and, more precisely, intelligent proportional controllers. This setting, which has been successfully tested in many concrete situations, has already been illustrated via various questions about intelligent transportation systems (see, e.g., [12]}, [13]}, [14]}, [15]}, [16]}, [17]}, [18]}, [19]}, [20]}). Concrete experiments show today that model-free control not only yields better traffic regulation than ALINEA but is also simpler to implement.There are many variants of ALINEA in the literature. We have selected for our computer comparisons the version [21]}.

